At least three security personnel and a civilian have been killed in the port town of Trincomalee in north-eastern Sri Lanka, officials say.
The civilian died in violence after two mine blasts killed the security men.
Recent weeks have seen frequent attacks against security forces in the city and rising tension between Tamil and Sinhalese communities.
The latest violence comes a day after Tamil Tiger rebels said they would not take part in peace talks due next week.
The Tigers said their decision came after a recent escalation in violence in Tamil-majority areas.
The Sri Lankan government said on Friday that it "deplored" the refusal by the Liberation Tigers of Tamil Eelam (LTTE) to participate in talks.
"The attitude of the LTTE is all the more regrettable in the wake of a series of unprovoked bomb attacks," a government statement said.
Surge in violence
More than 60 people have died in bombings in Sri Lanka in the past week.
The rebels say at least 10 Tamils were killed earlier this week by troops and paramilitary groups - a charge the military denied.
The army accuses the Tamil Tigers of being behind attacks on the security forces. The rebels in turn deny this.
The rebels said on Thursday they would not set a new date for talks until "normality returns".
The head of the international ceasefire monitors, Ulf Henricsson, travelled to rebel headquarters at Kilinochchi in the north on Friday seeking to iron out obstacles to the peace talks.
Correspondents say the past week has been one of the bloodiest since the government and the rebels agreed a 2002 truce.
The peace talks had been due to take place on 24-25 April in Geneva.
Reviving Sri Lanka's stalled negotiations is considered vital to strengthen a battered four-year ceasefire.
A surge in attacks in the north and east at the turn of the year abated after agreement was secured to hold talks in February, but violence has risen again in recent weeks.
The Tigers want self-rule for minority Tamils in the north and east of Sri Lanka. More than 60,000 people have died during two decades of conflict.
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